1. Field of the Invention
This invention relates to wideband signal transmission used for example in main line security systems or in a television studio video control grid, or in a star structured videocommunication network.
2. Description of the Prior Art
In particular, in a videocommunication network, numerous switching network are necessary for the selection of programmes by a user and to assure videophone communications. Switching network are also necessary in the head end equipment of the videocommunication network. In order to provide services such as regional or national broadcasting of television programmes from wherever they are produced, or also, to bridge different videocommunication networks. The switching network environment now consists of optical fibers for user access, of carrier current or digital transmission systems with corresponding standard interfaces and base band video interfaces. Signals to be switched might be, depending on the case, base band video signals, modulated video signals, digitalized video signals having a bit rate of the order or 100 Mbit/s for distributing in a local network, standard digital multiplexed signals having a bit rate of between 2048 Kbit/s and 140 Mbit/s, for example.
Wideband signal transmission through transmission line having balanced conductor pairs has the advantage over coaxial cables of possible economies in the transmission medium, easy implementation thanks to insulator piercing mass connections, and strong protection to electromagnetic interference if balancing between the two conductors in each line is rigorously maintained.
The latter advantage is especially difficult to achieve over the whole spectrum of a base band video signal between approximately 6 Hz and 6 MHz. Conventional techniques based on the use of transformers alone or only on active circuits are no longer applicable. Effectively, easily set up circuits require
transformers operating for audiofrequencies included in a low frequency range between 20 Hz and 20 KHz,
or, wideband transformers covering a limited number of decades, for examples between 100 KHz and 100 MHz,
or, active circuits using integrated circuits of differential or operational amplifiers.
These active circuits are difficult to use outside a frequency range of a few hundred KHz if one wishes to achieve a rigorous balancing that can be achieved with transformers.